1. Field of the Invention
This invention relates to a squeeze type pump for transferring slurry such as freshly mixed concrete. More particularly, the invention relates to a structure for improving the durability of a tube forming a passageway for slurry and also preventing pulsation resulting from the squeezing of slurry.
2. Description of the Related Art
Generally, as shown in FIG. 12, a squeeze type pump 70 for transferring slurry such as freshly mixed concrete is mounted on a vehicle 71. The squeeze type pump 70 is driven by a drive source 72 such as a motor. As the pump 70 is driven, the slurry is introduced into a hopper 73, transferred through a transfer tube 74, and discharged from a discharge port 75 of the distal end of the transfer tube 74 to a work location such as a building, etc. A boom 76 is also mounted to the vehicle 71. As shown by the broken chain line in FIG. 12, as the boom 76 is expanded, the transfer tube 74 is extended upward. Thus, the slurry that is transferred through the transfer tube 74 can be supplied to a higher location.
In the squeeze type pump described above, an elastic tube 52 is disposed along the inner circumferential surface of a drum 53, as shown in FIG. 11. A pair of pressing rollers 50 and 51 are rotated on a center of axis .alpha. of the drum 53 in the direction indicated by the arrow, while pressing the tube 52 against the inner circumferential surface of the drum 53. Consequently, the slurry in the tube 52 is squeezed and pumped.
When the leading roller 50 completes the squeeze operation and starts moving away from the tube 52, a chamber 61 of the tube 52 between both rollers 50 and 51 is communicated with a chamber 60 on the downstream side of the leading roller 50. In the initial stage of this movement, the roller 50 is not yet fully separated from the tube 52 and a narrow gap is formed inside the tube 52 at a position where the roller 50 contacts the tube 52.
The internal pressure of the chamber 60 is higher than that of the chamber 61 between the rollers 50 and 51. For this reason, when the above-described gap is formed, the slurry in the chamber 60 will abruptly flow backward through the gap into the chamber 61. This back flow causes pulsation of the slurry which severely wears the inner surface of the tube 52. In addition, the capacity of the pump is reduced because of the back flow of the slurry.
In order to overcome the above-described problem, there has been proposed a squeeze type pump disclosed in Japanese Unexamined Patent Publication No. 52-149605. See FIG. 8. The tube 52A of this pump is tapered. The cross-sectional area of the passageway is large at the upstream side and small at the downstream side.
In this pump, when both rollers 50 and 51 are moved from a position on line 3--3 to a position on line 4--4, the volume of a chamber 61A located between the rollers 50 and 51 is gradually reduced and thus the pressure in the chamber 61A is gradually increased. Therefore, when the leading roller 50 is moved away from a tube 52A and the chamber 61A is communicated with a chamber 60A, there will be little or no back flow of the slurry in the chamber 60A toward the chamber 61A because of the relatively high pressure in the chamber 61A. As a consequence, pulsation hardly occurs, pump operation is relatively smooth, and wear on the inner surface of the tube 52A is reduced.
As shown in FIG. 9, there has also been proposed a pump in which the diameter of the central portion of each of rollers 50A and 51A is made larger than that of the end portions. In such a pump, when a tube 52 is deformed into a flat shape, the pressing force acting on the laterally opposite curved portions 56 of the tube 52 is relatively lower. As a result, the fatigue on the curved portion 56 is reduced and the durability of the tube 52 is improved.
However, in the above-described pump, when the tube 52 is deformed into a flat shape between the inner circumferential surface of the drum 53 and the rollers 50A, 51A, it is possible that the tube 52 may be dislocated right or left because of a difference in densities of the slurry in the tube 52. For example, if the tube 52 is dislocated left as shown by the broken line in FIG. 9, a gap 55 will occur in the inside of the curved portion 56 of the tube 52. In such case, when the rollers 50A and 51A are located at positions corresponding to the positions on line 5--5 of FIG. 8 where the chamber 61A is not yet completely formed, the relatively high-pressure slurry in the chamber 60A flows backward through the gap 55. As a result, the inner surface of the tube 52 is severely worn and at the same time the capacity of the pump is reduced.
In U.S. Pat. No. 4,730,993, a squeeze type pump is disclosed in which the opposite sides of a tube 52 are pressed by a pair of parallel spaced rollers 57, as shown in FIG. 10. When a tapered tube such as that shown in FIG. 8 is used in this pump, a central portion 65 of an upstream section of the tube 52 is pressed by the central portions 64 of the lower rollers 57. Since the tube 52 is centrally pressed, no gap is formed in the pressed portion 54 of the tube 52.
However, in the downstream stage of the squeeze by a pair of upper rollers 57 shown in FIG. 10, the central portion 65 of the tube 52 is outwardly spaced from the center 64 of the rollers 57. This dislocation of the tube 52 in the downstream stage of the squeeze is due to the pressing of the tube 52 toward the drum 53 when the rollers 57 are rotated. For this reason, the gap 55 is produced in the outward portion of the pressed portion 54 of the tube 52. As a result, back flow of the slurry, wear on the inner surface of the tube 52, and a reduction in the pump capacity occurs. In addition, the durability of the tube 52 is reduced since a great compression force is applied to the inner curved portion of the tube 52 opposite the gap 55 by the large-diameter portions of the rollers 57.